Electroplating Process for Using Trivalent Chromium Electroplating Solution

ABSTRACT

A trivalent chromium electroplating solution in accordance with the present invention contains at least one trivalent chromium salt for electroplating a chromium coating layer on a workpiece. By using the low toxic trivalent chromium to substitute highly toxic hexavalent chromium, an electroplating process of the present trivalent chromium electroplating solution has less pollution.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. patent application Ser.No. 11/654,265, filed on Jan. 17, 2007, titled Trivalent ChromiumElectroplating Solution and an Electroplating Process with the Solution,listing Ching-An Huang, Chun-Ching Hsu and Ui-Wei Leu as inventors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electroplating bath contained withlow toxic trivalent chromium ions to obtain chromium deposit throughelectroplating.

2. Description of the Prior Art

Because of high hardness, excellent wear and oxidation resistance, thechromium deposit is commonly used in industrial applications. Nowadays,the chromium deposit is mainly obtained from the plating bath containinghexavalent chromium ion which is highly toxic in nature. Thus, todevelop alternative surface coatings for replacement of hard anddecorative chromium deposits has been much attracted in recent years.The plating bath with low toxic trivalent chromium ions could be asuitable alternative electroplating method for obtaining chromiumdeposit. However, lack of stability and difficult to obtain thickchromium deposit are presently the main drawbacks for trivalent chromiumelectroplating. In this invention, a stable trivalent chromiumelectroplating bath and its electroplating process will be stressed.

SUMMARY OF THE INVENTION

The main objective of this invention is to provide a low toxic trivalentchromium electroplating bath, which can be used to achieve chromiumdeposit by electroplating with high current efficiency and highelectroplating current density; meanwhile, an electroplating process forusing the trivalent chromium electroplating solution is provided.

To achieve foregoing main objective, the trivalent chromiumelectroplating bath comprises an aqueous solution added with trivalentchromium salt, a complex agent, conductive salt, a buffering agent, andan additive, wherein the additive is a mixture of ammonium bromide,sodium bromide, and potassium bromide.

The electroplating process comprises the steps of: (a) degreasing theworkpiece; (b) washing the workpiece; (c) surface activating to enhancethe binding ability between the surface and workpiece; (d) trivalentchromium electroplating on the workpiece; and (e) post drying.

Benefits and advantages of the present invention will become apparentafter a careful reading of the detailed description with appropriatereference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 demonstrates the flow chart of an electroplating process usingthe trivalent chromium electroplating solution in accordance with thepresent invention;

FIG. 2 shows a SEM (Scanning Electron Microscopy) micrograph of achromium layer on steel surface by using above-mentioned electroplatingprocess with the trivalent chromium electroplating solution;

FIG. 3 is an EDS (Energy Dispersive X-ray Spectrometer) spectrum of thechromium layer obtained from the electroplating process with thetrivalent chromium electroplating bath;

FIG. 4 is a SEM micrograph of cross-sectional chromium layer obtainedfrom the electroplating process with the trivalent chromiumelectroplating solution; and

FIG. 5 presents the figure showing the current efficiency of thetrivalent chromium electroplating at different temperatures andelectroplating current densities.

DETAILED DESCRIPTION OF THE INVENTION

A trivalent chromium electroplating solution in accordance with thepresent invention contains at least one trivalent chromium salt forelectroplating a chromium deposit on a workpiece. To obtain the chromiumdeposit on a workpiece, electroplating is conducted with the bathcontaining low toxic trivalent chromium ions to substitute the bath withhighly toxic hexavalent chromium ions. Therefore, the electroplatingprocess of the present trivalent chromium electroplating bath is anenvironmental friendly method.

Owing to its high hardness, superior oxidation and wear resistance,chromium coating is widely used for surface finish of precisionmechanical parts, moulds, and surgical tools. According to thethickness, two types of chromium deposits could be classified. The firsttype is decorative chromium deposit and the other type is hard chromiumdeposit. The decorative chromium deposit usually has a thickness ofabout 0.1-0.5 μm. The decorative chromium is mainly used for decorationand protection because of its varnish appearance. The hard chromiumdeposit has a thickness higher than 1 μm and is used for wear andcorrosion resistance. The trivalent chromium electroplating bath and itselectroplating process in the present invention can be applied to obtainboth the decorative and hard chromium deposits through electroplating.

The trivalent chromium electroplating solution substantially comprises:

(a) trivalent chromium salt: 0.2-1.4 mole/L of trivalent chromium saltthat is selected from the group consisting of chromium chloride,chromium sulfate and hydrates of the foregoing components;

(b) complex agent: 0.2-4.2 mole/L of the complex agent that is selectedfrom the group consisting of urea (carbamide), glycine (aminoaceticacid), hydroxyacetic acid, formic acid, dissoluble salts of acids, anddissoluble salts of urea;

(c) conductive salt: 1.0-3.0 mole/L of the conductive salt componentthat is a mixture having at least two components selected from the groupconsisting of ammonium chloride, sodium chloride, potassium chloride,magnesium chloride, ammonium sulfate, sodium sulfate, potassium sulfate,and magnesium sulfate;

(d) buffering agent: 0.1-0.8 mole/L of the buffering agent that isselected from the group consisting of boric acid, aluminum chloride,aluminum sulfate, and hydrates of foregoing components; and

(e) additive: 0.01-0.25 mole/L of the additive that is selected from thegroup consisting of ammonium bromide, sodium bromide, potassium bromide,and a mixture of the foregoing components.

According to above description, some preferred compositions of thetrivalent chromium electroplating solution are shown in the followingtable:

component Complex Conductive salt Buffering Num. Cr⁺³ agent compositionagent Addative 1 1.2 mole/L 4.8 mole/L 1.5 mole/L 0.8 mole/L 0.3 mole/L2 0.1 mole/L 0.4 mole/L 1.0 mole/L 0.4 mole/L 0.05 mole/L  3 0.2 mole/L2.4 mole/L 2.0 mole/L 0.6 mole/L 0.25 mole/L  4 0.8 mole/L 0.4 mole/L1.0 mole/L 0.5 mole/L 0.05 mole/L  5 0.6 mole/L 4.0 mole/L 1.2 mole/L0.8 mole/L 0.2 mole/L 6 0.8 mole/L 2.4 mole/L 1.5 mole/L 0.8 mole/L 0.1mole/L 7 1.1 mole/L 3.8 mole/L 2.4 mole/L 0.8 mole/L 0.3 mole/L 8 0.6mole/L 1.2 mole/L 1.2 mole/L 0.6 mole/L 0.05 mole/L  9 0.5 mole/L 2.4mole/L 1.8 mole/L 0.8 mole/L 0.1 mole/L 10 1.0 mole/L 1.8 mole/L 1.5mole/L 0.6 mole/L 0.2 mole/L 11 0.9 mole/L 0.5 mole/L 0.8 mole/L 0.4mole/L 0.15 mole/L  12 0.4 mole/L 0.8 mole/L 1.2 mole/L 0.7 mole/L 0.05mole/L  13 1.6 mole/L 5.4 mole/L 1.2 mole/L 0.8 mole/L 0.3 mole/L 14 0.7mole/L 2.4 mole/L 0.8 mole/L 0.3 mole/L 0.15 mole/L  15 0.5 mole/L 1.05mole/L  1.0 mole/L 0.8 mole/L 0.1 mole/L 16 1.4 mole/L 4.2 mole/L 3.0mole/L 0.8 mole/L 0.2 mole/L 17 0.6 mole/L 3.6 mole/L 2.1 mole/L 0.8mole/L 0.1 mole/L 18 0.3 mole/L 0.6 mole/L 1.5 mole/L 0.7 mole/L 0.05mole/L  19 1.5 mole/L 1.5 mole/L 1.5 mole/L 0.8 mole/L 0.1 mole/L 20 1.1mole/L 3.0 mole/L 2.4 mole/L 0.7 mole/L 0.25 mole/L  21 0.7 mole/L 1.8mole/L 1.8 mole/L 0.8 mole/L 0.15 mole/L  22 1.3 mole/L 0.8 mole/L 1.0mole/L 0.5 mole/L 0.25 mole/L  23 0.4 mole/L 2.4 mole/L 1.8 mole/L 0.6mole/L 0.1 mole/L 24 0.9 mole/L 1.35 mole/L  1.2 mole/L 0.8 mole/L 0.15mole/L  25 1.0 mole/L 4.0 mole/L 2.4 mole/L 0.6 mole/L 0.1 mole/L

Compositions of the trivalent chromium electroplating solution in numberof 6, 8, 10, 12, 15, 18, 21 and 24 in the table are mostly preferred.

With reference to FIG. 1, an electroplating process is demonstrated andadopted to electroplate a chromium deposit on a workpiece, for exampleharden high-carbon tool steel, by using the trivalent chromiumelectroplating solution in the present invention. The electroplatingprocess comprises steps of:

(a). degreasing:

The prepared workpiece is degreased with a degreasing agent to removeoil and dirt from its surface.

(b). washing:

The prepared workpiece is washed to remove the degreasing agent and tokeep the surface clean.

(c) surface activating:

The workpiece is dipped into acid or alkaline solution to activate thesurface by adding oxidant or providing electricity so as to enhance thebinding efficiency between the surface of the workpiece and the chromiumdeposit.

(d) trivalent chromium electroplating:

The workpiece, auxiliary electrodes, and the trivalent chromiumelectroplating solution are set in a cell tank. A fixed current isapplied across the workpiece and the auxiliary electrodes by anadditional power supply to start electroplating operation. No extramembrane, for example Nafion, is necessary to separate electroplatingsolutions between cathode and anode.

(e) drying:

After electroplating, residual electroplating solution is removed fromthe surface of the workpiece by clean agent dipping and hot-air blasterdrying.

Wherein, the auxiliary electrodes are made of material selected from thegroup comprising with the platinized Ti mesh or plate, platinum,graphite and stainless steel.

Wherein, the fixed current provided by the additional power supply has arange from 5 to 95 ampere per square decimeter.

Wherein, the temperature for the trivalent chromium electroplating onthe workpiece is in the range of 1 to 60° C.

As presented in FIG. 1, a preferred flowchart of an electroplatingprocess to achieve the trivalent chromium deposit is shown. Beforeelectroplating of trivalent chromium, the workpiece is prepared (10),degreased (11) and washed (12) to make its surface clean. Then, thesurface of the workpiece is activated (13) and washed (14) again. Afterinputting the workpiece, auxiliary electrodes, and the trivalentchromium electroplating solution into a tank, electroplating ismechanically stirred (15). By applying a fixed current from anadditional power supply, the ions of chromium complex obtain electronsto reduce and deposit on the surface of the workpiece so that theworkpiece obtains a uniform chromium deposit. After electroplating, theworkpiece is washed (16) again and dried (17) to obtain the finalproduct.

With reference to FIGS. 2, 3 and 4, the final product is observed byScanning Electronic Microscopy (SEM) and Energy Dispersive X-raySpectrometer (EDS) to exam the surface and a cross-section of thedeposited workpiece. According to those figures, the workpiece indeedobtains a uniform chromium deposit on the surface thereof.

With reference to FIG. 5, the graph shows that deposition rate of thechromium coating layer is influenced by the temperature and the platingcurrent density.

In summary, the trivalent chromium electroplating bath is an aqueoussolution added with a trivalent chromium salt, a complex agent, aconductive salt, a buffering agent, and an additive, wherein theadditive is a mixture of ammonium bromide, sodium bromide or potassiumbromide.

Wherein, the additive is of 0.01 to 0.25 mole/L in the trivalentchromium electroplating solution.

Wherein, the buffering agent is selected from the group consisting ofboric acid, aluminum salt, dissolvable salt of foregoing components, amixture of at least two of the foregoing components, and the dissolvablesalt.

Wherein, boric acid and/or the dissolvable salt are of 0.1-0.8 mole/L inthe trivalent chromium electroplating solution.

Wherein, the aluminum salt is selected from the group consisting ofaluminum chloride, aluminum sulfate, and hydrates of the foregoingcomponents.

Wherein, the aluminum salt and/or the hydrates of the aluminum salt areof 0.1-0.6 mole/L in the trivalent chromium electroplating solution.

Wherein, the complex agent is selected from the group consisting of urea(carbamide), glycine (aminoacetic acid), formic acid, dissoluble saltsof urea, and dissoluble salts of acids.

Wherein, urea and/or its dissoluble salt are of 0.2-4.2 mole/L in thetrivalent chromium electroplating solution.

Wherein, glycine and/or its dissoluble salt are of 0.1-2.8 mole/L in thetrivalent chromium electroplating solution.

Wherein, hydroxyacetic acid and/or its dissoluble salt are of 0.1-2.8mole/L in the trivalent chromium electroplating solution.

Wherein, formic acid and/or its dissoluble salt are of 0.1-1.4 mole/L inthe trivalent chromium electroplating solution.

Wherein, the conductive salt composition is a mixture having at leasttwo components selected from the group consisting of ammonium chloride,sodium chloride, potassium chloride, magnesium chloride, ammoniumsulfate, sodium sulfate, potassium sulfate, and magnesium sulfate.

Wherein, the conductive salt composition is of 1.0-3.0 mole/L in thetrivalent chromium electroplating solution.

Wherein, the trivalent chromium salt is selected from the groupconsisting of chromium chloride, chromium sulfate, and hydrates of theforegoing components.

Wherein, the trivalent chromium salt and/or its hydrate are of 0.2-1.4mole/L in the trivalent chromium electroplating solution.

Although the invention has been explained in relation to its preferredembodiment, it is to be understood that many other possiblemodifications and variations can be made without departing from thespirit and scope of the invention as hereinafter claimed.

1-12. (canceled)
 13. An electroplating process for using a trivalentchromium electroplating solution comprising: (a) degreasing, wherein aprepared workpiece is degreased with a degreasing agent to remove oiland dirt from the surface of the workpiece; (b) washing, wherein theprepared workpiece is washed to remove the degreasing agent and to keepthe cleaned surface of the workpiece; (c) surface activating, whereinthe workpiece is dipped into one of acid and alkaline solution toactivate the surface by adding oxidant or providing electricity so as toenhance the binding efficiency of the surface of the workpiece; (d)trivalent chromium electroplating, wherein the workpiece, auxiliaryelectrodes, and the trivalent chromium electroplating solution are setin a tank and a fixed current is applied across between the workpieceand auxiliary electrodes by an additional power supply to startelectroplating operation; and (e) drying.
 14. The electroplating processas claimed in claim 13, wherein the auxiliary electrodes are made ofmaterial selected from the group comprising plantized titanium mesh,titanium plate, platinum, graphite, and stainless steel.
 15. Theelectroplating process as claimed in claim 13, wherein the fixed currentprovided by the additional power supply has a range from 5 to 95 ampereper square decimeter.
 16. The operational method as claimed in claim 13,wherein the temperature for a surface treatment of the workpiece is in arange of 1 to 60° C.
 17. The electroplating process as claimed in claim13, wherein the trivalent chromium electroplating solution comprises anaqua solution added with trivalent chromium salt, a complex agent,conductive salt, a buffering agent, and an additive, wherein theadditive is a mixture of ammonium bromide, sodium bromide and potassiumbromide.
 18. The electroplating process as claimed in claim 17, whereinthe trivalent chromium salt is selected from the group consisting ofchromium chloride, chromium sulfate, and hydrates of the foregoingcomponents; and the trivalent chromium salt and a hydrate of thetrivalent chromium salt are of 0.2-1.4 mole/L in the trivalent chromiumelectroplating solution. 19-24. (canceled)
 25. The electroplatingprocess as claimed in claim 17, wherein the trivalent chromium salt isselected from the group consisting of chromium chloride, chromiumsulfate, and hydrates of the foregoing components; one of the trivalentchromium salt and a hydrate of the trivalent chromium salt is of 0.2-1.4mole/L in the trivalent chromium electroplating solution.